Peptide fragments are being isolated from digests of the unique isozyme of lactate dehydrogenase (LDH-X; LDH-C4) found in mature testes of spermatozoa of most mammals and some birds. These peptides of known amino acid sequence are being tested for their capacity to bind antibody to the native protein. Antibody binding peptides will be conjugated to appropriate carriers to determne if they can provoke antibody specific to determinants in LDH-C4.